How to change the thickness of grass hairs?
Mate i have a better idea for u, A way that I like to create Grass is using Planes, Not cylinders because they use way to many faces. The Goal in 3D art is to create as little faces as possible whilst still achieving a great Render. Try and create a narrow plane and scale it along the z-Axis then extrude it to make it look like a strand of Grass. Then duplicate it about 15 times moving them in different Locations to make it look like a Grass Blob. Add it to a Group. Then go to your Plane section and add a Vertex group. Go to weight paint and paint where you want the grass to appear. Then add Particle System/ Hair and click advanced. Scroll down to where it says group and click the object you added to a group. (The grass) There you can change the size and brownian to make it look like Grass.
And viola, you get an awesome scene that you can change.